Some engines may be configured to perform what may be referred to as controlled auto-ignition (CAI), whereby a mixture including air and fuel is auto-ignited during a compression stroke of the cylinder's piston without necessarily requiring a spark or a pilot injection to initiate combustion. One particular type of controlled auto-ignition known as homogeneous charge compression ignition (HCCI) includes auto-ignition of a substantially homogenous mixture of air and fuel. HCCI may be used to achieve improved engine efficiency and reduced emissions, under some conditions. However, under other conditions, it may be difficult to achieve auto-ignition or to control the timing of auto-ignition. For example, during certain higher or lower engine torque or engine speed ranges, auto-ignition may be difficult to control resulting in misfire, engine knock, or pre-ignition.
One approach to address this issue includes the use of charge temperature control to extend the operating range of auto-ignition. As one example, exhaust gases may be recirculated from the exhaust manifold to the intake manifold via an external exhaust gas recirculation (EGR) passage. These EGR gases may be used to provide charge heating, whereby the amount of EGR gases supplied to the cylinders may be adjusted to control the timing of auto-ignition. In this way, HCCI mode operation may be extended. However, the inventors of the present disclosure have recognized that this approach utilizes additional hardware including an EGR passage, EGR valves, and additional control systems, thereby increasing the cost or complexity of the engine system. As another example, a portion of the exhaust gases retained by each cylinder may be controlled by varying the timing of an exhaust valve of the cylinder. However, the inventors herein have recognized that this approach may still not provide sufficient charge heating during some conditions. For example, during lower engine load conditions, the cylinder may not retain sufficient heat to enable auto-ignition.
Another approach that may be used to extend HCCI operation to lower load or torque ranges includes the practice of deactivating some of the engine cylinders, thereby increasing the effective load at the active cylinders. However, the inventors have recognized that this approach may result in increased noise and vibration harshness (NVH), under some conditions.
In order to address some of these and other issues, the inventors herein have provided a method of operating an engine. The engine includes at least one cylinder communicating with an intake manifold via an intake manifold valve and an exhaust manifold via an exhaust manifold valve, the cylinder including a piston arranged within the cylinder, wherein the piston is coupled to a crankshaft of the engine. The method comprises discontinuing combustion in the cylinder during a plurality of cycles of the engine; during the plurality of cycles, operating the exhaust manifold valve and the intake manifold valve to provide a net flow of gases from the exhaust manifold to the intake manifold via the cylinder and adjusting a torque signature provided to the crankshaft during each cycle by the piston responsive to an operating condition.
In this way, exhaust gases produced by the first cylinder may be returned to the intake manifold via at least a second cylinder where they may be used to provide charge heating for subsequent combustion events by the first cylinder while the second cylinder can provide a torque signature to the crank shaft. Thus, NVH may be reduced or engine braking may be performed, for example, during a deactivated state of the second cylinder. Note that combustion in the first cylinder is not limited to auto-ignition, and that other types of combustion may be performed by the first cylinder.